The present invention relates generally to amplifier circuits and more particularly to a mode selection amplifier circuit usable in a signal acquisition probe.
Signal acquisition probes acquire various types of electrical signals from a device under test. Generally, electrical signals may be divided into single ended signals and differential signals. The single ended signal is carried on a signal conductor that has an associated reference, generally electrical ground. The differential signal is carried on a pair of signal conductors with the differential signal having two electrical signal components.
Single ended and differential voltage signal acquisition probes are manufactured to measure the respective single ended and differential signals. The voltage single ended signal acquisition probe has a probing tip for contacting a test point on a device under test carrying the single ended signal. A ground contact or lead extends from the signal acquisition probe for connecting to a ground reference associated with the signal ended signal. For single ended active voltage probes, the probing tip is electrically coupled to buffer amplifier via an attenuator circuit. The buffer amplifier isolates the signal from loading by the probe cable and provides a controlled impedance output. The output of the buffer amplifier is coupled to a probe signal cable which generally has a 50 ohm impedance characteristic. The other end of the cable is coupled to a measurement test instrument, such as an oscilloscope or the like, via a probe control box for displaying a representation of the signal on a display device of the oscilloscope.
The differential voltage signal acquisition probe has dual probing tips for contacting test points on the device under test carrying the differential signal. The dual probing tips are coupled to inputs of a differential amplifier via a dual attenuator circuit. The differential amplifier generates an output that is proportional to the differential signal. The output of the differential amplifier is coupled to a probe signal cable which generally has a 50 ohm characteristic impedance. The other end of the cable is coupled to measurement test instrument, such as an oscilloscope or the like, via a probe control box for displaying a representation of the difference of the signal components of the differential signal on a display device of the oscilloscope.
The differential signal has common mode components that are measured with two single ended voltage signal acquisition probes. The probing tip of one of the single ended voltage signal acquisition probe is coupled to one of the signal components of the differential signal and the probing tip of the other single ended voltage signal acquisition probe is coupled to the other signal component of the differential signal. Each signal acquisition probe is coupled to separate signal input channels of the oscilloscope where the oscilloscope sums the signal components and divides the summed result by 2 to generate a common mode signal result. The common mode signal result is displayed on the display device of the oscilloscope.
Measuring the common mode voltage of a differential signal using two single ended signal acquisition probes has a number of drawbacks. Gain or delay mismatches between the probes can cause inaccuracies in the common mode measurement. To reduce these delays, the probes need to be deskewed so as to align the signal from one probe with the other. This requires the use of a deskew fixture and algorithms within the measurement test instrument for aligning the signals from the deskew fixture. Any internal delays in the instrument that have not been calibrated out of the signal channels will affect the common mode measurement.
What is needed is an amplifier circuit within the signal acquisition probe that permits the selection of various measurement modes. The amplifier circuit should be able to receive voltage signals that are referenced to a reference source, such as electrical ground, and differential voltage signals and generate various signal output modes that include differential and common mode signal outputs as well as voltage signals referenced to a reference source. The amplifier circuit should be usable in a signal acquisition probe configurable as both a single ended and a differential signal acquisition probe.